A lithium ion battery, as a chargeable high capacity battery, enabled an electronic device to have a higher function and to operate over a long time. Furthermore, the lithium ion battery is installed in vehicles and is considered as a promising battery of hybrid vehicles and electric vehicles. The lithium ion battery, which is now used widely, includes an active material such as lithium cobaltate; a positive electrode formed by applying a paste containing a binder such as polyvinylidene fluoride (PVDF) on an aluminum foil; and a negative electrode formed by applying a paste containing a carbon based active material and a binder such as PVDF or a styrene-butadiene-rubber (SBR) on a copper foil.
In order to further increase a capacity of the lithium ion battery, use of silicon, germanium or tin as a negative electrode active material has been studied (see, for example, Patent Literature 1). Since the negative electrode active material using silicon, germanium or tin can receive lots of lithium ions, the active material undergoes a large change in volume between the time when sufficient charging is performed and the time when sufficient discharging is performed, and thus it is impossible for the binder such as PVDF or SBR to follow the change in volume of the active material. Therefore, use of a polyimide resin having more excellent mechanical characteristics as a binder of a negative electrode has been studied (see, for example, Patent Literature 2).
Use of such novel high capacity active material may often cause discharging of electricity only in the quantity smaller than that in the case of charging, when discharging is performed after initial charging. Therefore, a study is now made on a method in which an improvement is made by adding lithium first (see, for example, Patent Literature 3).
There is also a problem that use of a polyimide resin as a binder causes low first time efficiency. To cope with the problem, there is disclosure that use of a mixture of two kinds of polyimide resins each having a different molecular weight, for example, a polyimide resin having a molecular weight of less than 100,000 and a polyimide resin having a molecular weight of 100,000 or more and less than 200,000, enables the production of a binder capable of maintaining point binding properties and strength, and thus increasing initial efficiency without causing deterioration of current collection characteristics (see, for example, Patent Literature 4).